Transhission circuits



Dec. 31, 1929.

H. WHITTLE TRANSMI SS I ON C IRCU Filed Sept. 28, 1927 ITS 2A Sheets-Sheet 1 f/G. 2. I 7 //5 8 I) /2 'v /f /6 3 T6] I TC, e

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HORA 0E WH/mE Dec. 31, 1929. H.- wHlTTLE TRANSMISSION CIRCUITS Filed Sept. 28, 1927 2 Sheets-Sheet FREQUENCY .H5 En S...

E Hm mm MW VE WG BY j l ing frequencies Patented Dec. 31', 1929 A PA'riiNT OFFICE nonnen WnrrrLE, or MAPLEwoon,

-Lasoim'romas,l INCORPORATED, or N Yom;

NEW JERSEY, ASSIGNOR '.10 BELL TELEPHONE EW YORK, N. Y., A CORPORATION OF NEW 'rnANsMrssIoNcmcUITs appiicatian and sepfeniber 2s, 1927. serial No. 222,438.`

1`liis invention relates to transmission circuits and particularly to such circuits in which amplifying .devices are employed.

' vAn object of the inventionis to improve the amplification-frequency characteristics of an amplifying circuit.- ,l f

A further Objectis to extend the range of signaling frequencies whichmay .be amplified without appreciable distortion by an amplifying circuit for a given'amount of amplification therein.

Afurther object is to increase the voltage obtained in an amplification which may be.

l-range of freamplifier circuit for agiven queiicies transmitted and a given standardof qualityof transmission. 1

A feature of the invention is a multi-stage vacuum 'tube amplifier circuit havinga tuned input circuit, the tuning of which is auto niatically controlled in accordance with the frequency of incoming waves.

The above objects are accomplished in a multi-stage vacuum tube amplifier circuit including an input transformer and an inter. stage transformer by designing the interstage transformer to workon the leakage resonance point ot' its impedance characteristic for the rmaximum frequency at which voltage amplification is desired in the input transformer.

More specifically stated, the invention comprises an amplifier circuit consisting in part of an input transformer, a sp'ace discharge device., an interstage. transformer and a second space discharge device connected in cascade, the interstage transformer havinginductance of such value as to resonate with the distributed capacity between its windings and the effective capacity of the vacuum tube into which it feeds at'a frequency near the upper end of the band of signalto be amplified by the input transformer. 'The interstage transformer will present a low impedance to the plate circuit of the preceding tube at the resonant frequency and a high impedance thereto at lower frequencies. This results in ad'ecrease in the effective input capacity, and therefore, an inleakage crease in the input impedance of the preceding tube, as the frequencies to be transmitted approach the resonant frequency. 'VlhiS 111- crease in the tube input impedance at the high frequencies results in an increase in the voltage amplification whicli may be obtained in the input transformer at the high end of the frequency band and thus permits the inp'ut transformer, and therefor. the amplifier circuit as a whole, to transmit a wider frequency band without appreciable distortion for a given voltage amplification, or the same frequency band with a greater voltage amplification. l

The invention will be better understood by reference to the following detailed description when read in connection with the ac` -companying drawing in which Fig. 1 shows schematically a multi-stage vacuum tube amplifier circuit 2 is a schematic showing of a simplified circuit equivalent to the circuit of Fig. 1; Fig. .-3 Ashows graphically the transmission and iinpedance characteristics of a transformer such as is used as theinterstage transformer in the circuit of Figs. 1 and 2; Fig. 4 shows sclieinatically a vacuum tube amplifier circuit embodying a modification of the invention; and Fig. 5 shows graphically the impedance and transmission characteristics of a portion of the circuit of Fig. 4.

In Fig. 1. a multi-stage amplifier circuit is shown connected between an incoming line 1 and an outgoing line 2 for impressing on the outgoing line in amplified forni Signals received over the incomingline from a source 3. The amplifier circuit comprises an input transformer 4 having a primary winding 5 and a secondary winding 6, a three-electrode vacuum tube amplifying device 7, an interstage. transformer 8 having a primary winding 9 and a secondary winding 10, a second three-electrode vacuum tube amplifying device 11 andan output transformer 12 having a primary winding 13 and a secondary winding 14, connected in cascade in the order given. The incoming line 1, the impedance of which is represented by the resistance 15, is connected across the primary winding 5 of input transformer 4, and the outgoing line 2, the impedance of which is represented by the resistance 16, is connected across the secondary wind ing 14 of the output transformer l2,

embodying the invention; Fig.

ion

networks in the simplified circuit of Fig. 2,

The space vdischarge amplifying devices 7 and 11 have the usual filament heating, space current supply and grid polarizing batteries.

To obtain the maximum width for the band of signaling frequencies which may be transmitted without distortion by the amplifier circuit described fora, givenamount'of amplification, or the maximum amplification for a given band width and a given standard of quality of transmission, in` accordance with the invention, the interstage transformer 8 and the input transformer 4 are designed in t-he manner explained below in connection with Figs. 2 and 3 of the drawing.

In Fig. 2 is shown a simplified circuit equivalent to the amplifier circuit of Fig. 1 in which, in the usual manner, each of the transformers 4, 8, and 12, is represented by a T network, the series inductance arms of which represent the leakage inductance of the transformer, and the shunt. inductance arm its mutual inductance, and -in which each of the amplifyingr tubes 5 and 6 with its input and output impedances is represented by a network, shown in a dotted box correspondingly marked, comprising a source of electromotive force c, or e2, a shunt resistance R, and shunt and series capacities C1 and C2, C1 being the grid-filament capacity of the tubeiand Cgb the effective grid-plate capacity of the- .tii e. l

As pointed out in the patent. to lV. L.Cas per 1,530,649, issued March 24, 1925, a trans-- former having leakage inductance, as represented by the series inductance arni of the T when working into a vacuum tube consisting chiefly of capacitive impedance', has iinpedance-frequency characteristics such as shown in Fig. 3 of the drawing.

As shown in Fig. 3, the impedance eliarac, teristic of such Ya transformer when viewed from its primary winding terminals, as in-A dicated by Z3 in Figs. 1 and 2, has aresistance component, as indicated by the curve Il, and a reartance component as'indicated byv the curve X.' The reactance component X of the impedance characteristic has a parallelA resonant portion and a series resonant portion. At the frequency indicated by the' point H on the curve X, the transformer frequency indicated by the point E on the curve X, the transformer 4passes through series resonance. If the leakage induetance cuit will be flat over a Wide range of frequenf cies, as shown by thecurve A of Fig. 3, bute flat characteristic is notessential to the at-v tainment ofthe results described 'in this invention. i

A transformer of the type ust described is employed as the interstage transformer 8 in the circuit of Fig. 1, the transformer^12 in the circuit of Fig. 1 being an ordinary output transformer designed -toj connect the output oftube 1-1 tothe load-.impedance represented -by the resistance 16l`. It.' is.'y desired'- to obtain as high a voltage amplification as possible forl the input transformer 4.. If input trans former 44 is designedto `have iniiiimum'dis- -tributed capacity between its` windings, it \may be shown as follows that the'ainoiint of voltage amplification which may be obtained therein is limited by the impedance looking from transformer 4 into the input circuit of 4tube 7, and the value.v of this'iiiipedance is limited bythe inputcapa'eity of tube 7.

If the input transformer 4 is assiuned to be working on the 'parallel resonance .point of Iits impedance eliara'ct'eristic.- the maximum voltage amplificationA ratio obtainable thereby .is whenthe impedance of the incoming line which we may designate Zi, as represented by the resistance 15in the circuits of Figs. 1 and 2 of theldrawing, times the impedance l ratio R of input transformer 4 is equal to the -anti-resonanceimpedance,whichwe may designateZ2, of 'the secondary winding G of this transformer withits distributed capacity and 1 the inputcapaeity of th'etube 7 into which it where fi-is the desired frequency to be transf initted on the center ofi the desired frequency range, C vthe tube c'apacityv plus the transformer capacity, .and :Q'the ratio of the re- 'actanee "towthe resistance of the secondary winding of the transformer at the anti-:reso- --naiit frequency.

' The voltage amplification under these conditionsequals lg-r. where R is theiin'pedaiice ratio, o r-l `the turns' ratio of the transformer.. A

The inputiinpedance 'Z2 is therefore limited ing on the leakage resonance point of its iin- Y penance cli`araeteristic` which usually occurs at its upper limiting frequency, the voltage amplification of the transformer is given by the following relation.

viv'hr th-voltage amplification is the ratio Y pedance ratio of transformer 4. N ow if C is decreased, it follows that the voltage amplification of transformer 4 is increased. Since the-'capacitive impedance of the tube and transformer increases as the -frequency diminishes itisin general possibleto maintain the same-voltage amplification at lower frequencies as occurs at the upper limiting frequency andto obtain-a substantially fiat characteristlc over a wide band of frequencies.

Now, it is well known that if a three-elecltrodevacuum tube amplifying device is shortcapacity parallel.

'i as circuitedin its output side, its input capacity 1s a1 m1nimu'm,since it is composed of gridfilamenti: capacity and effective grid-plate .andin'such ajcase, theeffectivel amplification factor pebf the tube is zero. On the other f hand, it isfalso `well known that if the plate circuit ofa three-electrode vacuum tube is open 4circuited vorfterx'ninated in a high impedance, the input/ capacity of the tube is greatly increased, since the effective amplification desirable to `o bt ain` a maxinlum voltage amplification withv a minimum amount of -Var1at1onm the amplification over a wlde lrange of frequencies. As shown in the above discussion, this may be accomplished bymaking the inputcvapacity of tube 7 a minimum at the highest frequencies to be transmitted.

' By designing the int-erstage transformer 8 so that it passes through leakage resonance at a frequency near the upper end of the band to be transmitted by the input transformer 4, the impedance'of the interstage transformer will be a minimum at this frequency, and will increase for the lower frequencies, as shown-by the curves of Fig. 3. The plate circuit of tube 7 is, therefore, effectively short-circuited for currents of the Aresonance frequency, but is terminated atotherlowerfrequencies by an impedance which decreases as the frequency increases. lAs brought out above, this causes the input capacity of tube 7 to decrease with increasing frequency, making its input-impedance more nearly constant msteadof vary-` ing inversely with frequency, This decrease lin capacity extends thewidthof the frequency.

band which may be transmitted-by the input transformer 4 with a given gain variation and thus the width -of -band'transmitted by the amplifier circuit as 'al whole.

.In an actual comparison of the results obtainable with the circuit of the invention with those obtainable with a-similar amplifier circuit but using resistance interstage coupling instead of. transformer coupling, it was found that the bandwidth transmitted by the input transformer with variation of 1 TU or less is increased about 15 per cent by using an interstage transformer in place of a resistance interstage coupling and by utilizing the effect of leakage resonance of the interstage transformer on the input capacity of the first tube. The circuit'of the invention may also be used for improving the impedance character# isticof the amplifier when viewed from the primary winding of transformer 4, An input transformer designed to give a uniform impedance characteristic usually operates atthe l shunt resonance point of its characteristic with a resistance bridged across the. primary terminals.4 As long as the coil impedance is high compared to this resistance, the effective impedance across these .terminals will be substantially the resistance of the shunt. Inv

the copending application of F. E. Field,

Serial No. 98,758, filed March 31, 1926, there is disclosed a transformer so designed that l its inductance willdeerease with increasing frequencies due to the screening effect on the flux of the eddy currents produced in the laminations of its core, thus approaching a condition of continuous parallel resonance, i. e. a resonance point which is automatically adjusted in accordance with the frequency with the distributed capacity of the transformer windings and the input capacity of the vacuum tube into which it works. Fig. 4 shows a multi-stage amplifier circuit em-i ploying an input transformer 4 having a resistance 17 bridged across its primary Winding andan interstage transformer 8 equivaient to that described in connection with the circuit of Fig. l. As the effect of the interstage transformer 8 is to cause the input capacity of the vacuum tube 7 to decrease with increasing frequency due to leakage resonance, indicated by the dotted variable capacity across the input yterminals of tube 7, and the effect of the \input transformer is to cause the inductance effectively in shunt with results in a very 'material improvement in the Y impedance characteristic of the input transcurves of Fig. 5. In the upper part of Fig.,5,

the solid curves 1' and X represent respectively the resistance component and reacta'ncc component of the impedance characteristic for the input transformer taken alone over a frequency range extending from a frequency f1 to a frequency f2, the latter frequency being the frequency at which the intersltage transformer passes through series resonance, and the dotted portions of the curves r and X indicate the change in the impedance characteristic due to the action of-the 4inters'tage transformer 8 in the circuit.` In the lower part of Fig. 5 the Solid curve A-is the amplification characteristic of the input transformer alone over the same. frequency range, and the dotted portion ofthe curve indicates the improvement in the amplification 'characteristic of the input transformer due to the action of the interstage transformer-8 in the circuit.

It is to be understood that this invention is not limitedv to .the particular form described above, but that it may .bevariously' modified Without departing'from vthe spirit of this invention as defined in the appended claims. Y

Vhat is claimed is:

1. In an electric discharge amplifier -having an input circuit and an output circuit, means in said output circuit for makingthe capacity of said input circuit a minimum, at substantially the upper limit. of a wide band of frequencies to be amplified. Y

2. In an electric discharge amplifier having an input circuit and an output circuit, for amplifying waves comprising a band of fre-- quencies, means in said output circuit for making the capacity of said input circuit a minimum for at least a part of the frequency band to be transmitted.

3. In an electric discharge amplifier having an input circuit and an output circuit, means in said output circuit for making the impedance of said input circuit a maximum at the upper limit of a wide band of frequencies to be amplified.

4. In an electric discharge amplifier having an input circuit and an output circuit, said input circuit comprising an input transformer havin a primary winding and a secondary win ing, means in said out ut circuitfor making the capacity of sai input circuit effectively in shunt to said secondary windino' a minimum for a frequency near the upper imit ofa wide bandof frequencies impressed on said input transformer.

5.A In a circuit for amplifying waves comprising a Wide band of frequencies, an input transformer supplied with saidtva'ves, aspa'ce discharge device fed thereb and an output circuit fed b said space cisc'harge device, means in sai output circuit for making the input impedance of said space discharge device amuximumv for a'frequency near the upper limit ofthe band to be amplified.

6. In a circuit for amplifying Waves comprising a Wide band of frequencies, an input transformer supplied with said Waves, a space discharge `device fed by said transformer, an output circuit fed by said space discharge device and means in said output circuit for making the input capacityof said space discharge device a minimum at a frequency near', the upper limit ofthe band to be amplified.

7. In combination, in -a circuit for ampli` fying waves comprising a Wide band of frequencies, an input transformer, a space discharge vdevice having an input circuit fed' thereby, and means automatically responsive ito change inthe frequencies to be amplied for controlling'the impedance of said input circuit so that said input transformer will produce on said waves a maximum amplificavtion for a given width of said band and a given amount of distortion therein.

i 8. 4In-combination in a circuit for amplifying waves comprising a ,wide band of frequencies, an input'transformer, a space ldischarge' device having an output circuit, and

an input' circuit offering substantially a capacitive impedance to Waves transmitted by said transformer, and means'in said output circuit automatically responsive to variations inthe frequency of said waves impressed on saidY input transformer to control said capacitive impedance so that it is a maximum for .the highest frequency in the band -to be amplified and remains substantially the same at somewhat lower frequencies in said. band.

9. In a circuit for-amplifying waves comprising a wide band of frequencies, an input transformer supplied with said waves,- a space discharge device having an input circuit and an output circuit, said input circuit being fed by said transformer, a second transformer i fed by said space discharge device, said second transformer vbeing lsuch as to offer minimum impedance in said output circuit to currents of a frequency near the upper limit of the band to be amplified, and a higher impedance therein to currents of the-lower frequencies in' said band.

l0. In a circuit for amplifying waves comprising a wideband of frequencies, an input transformer supplied with said waves, a space discharge device having aninput circuit and an output circuit, said input circuit being fed by said input. transformer, a second. transformer fed from said output circuit, said -second transformer feeding into a load circuit having a substantially*capacitive impedance, sald second transformer being designed with respect to said space discharge device and said load circuit to controlthe output yimpedance of ,said input transformer so as to effectively increase the width ofthe freuency band which may be transmitted t ereby for a given amount of amplification and a given amount of distortion.

11. 1n a system'for amplifyin electrical waves comprising a wide band of requencies, a circuit comprising an inputtransformer having a primary winding supplied with said waves and a secondary Winding, a space discharge device having an input circuit connected to said secondary winding, and an tial y capacitive impedance, a second transformer coupling the last-mentioned circuit to said output circuit, said second transformer offering a low impedance to currents .in said output circuit of a frequency near the upper end of said band and a'higher impedance to currents therein of the lower frequencies in said band, whereby the capacity of said input circuit looking from said input transformer is a minimum for transmitted 'waves of said frequency and increases for transmitted Waves of the lower frequencies in said band.

12.' In combindtion, a source of Waves comprising a wide band of frequencies, and a circuit for amplifyin said waves comprisin an input transformer and a secondary winding, said primary winding being connected to lsaid source, a space discharge device having an input circuit and an output circuit, sai input circuit bein connected to said secondary windin of sai input transformer, a second s ace `scharge device, an interstage trans ormer having windings coupling said output circuit of the first mentioned space discharge device to said second space discharge device, said interstage transformer having leakage inductance of such value as to resonate with the distributed capacity of its windings and the capacitive impedance of said second space discharge device at a frequenc near the upper end of said hand thereby o ering a low impedance to currents of said frequency in the output circuit of said first mentioned space discharge device and a higher impedance to currents therein of the lower frequencies in said band, whereby7` the input capacity and impedance of said lirstmentioned space discharge device is controlled to give said input transformer a maximum voltage amplification for a given width of frequency band and a given distortion inthe waves transmitted thereby.

13. A circuit for amplifying with substantially uniform eliiciency and for o'ering a substantially uniform Aim' edance to waves comprising a given band o frequencies, comprising an input `transformer havin an inductance which ,changes automatical with the frequency of the waves impressed t ereon and a shunt resistance across its primary output circuit, a circuit having a substan-` aving a primary win ing.

ductance which decreases as the frequency of the'waves supplied thereto increases, and a shunt resistance across its primary winding, a space discharge device having a substantially capacitive infput impedance fed thereby, an output circuit ed by said space discharge device, and meansv in ,said output circuit for making the input capacity of said space dislcharge device decrease as the frequency `of the Waves supplied thereto increases.

, 15. A circuit in accordance with claim 12 and in which said means comprises a second transformer coupling said output circuit to a capacitive load circuit, said second transformer being such as to offer a low impedance to currents of the higher frequencies in said band and a higher impedance to currents of the lower frequencies in said band.

16. A circuit in accordance with claim 12 and in which said means comprises a second transformer comprising windings coupling said output circuit to a capacitive load circuit, and having leakage inductanc'e of such value as to resonate with the distributed capacity of its windings and the capacitive impedance of said load circuit at a high frequenc in said'band and to cause said second trans ormer to produce in said output circuit a uniformly increasing impedance to currents therein of the lower fre uencies in said band.

In witness whereof, hereunto subscribe my name this 22nd day of September, A. D.

HORACE WHITTLE. 

